Temperature compensating (TC) capacitors exhibit a linear change in the temperature coefficient of capacitance (TCC) over the temperature range of -55.degree. C. and +125.degree. C. The different dielectrics employed have linear TCC slopes that vary between a negative slope of about 5,000 PPM/.degree. C. (parts per million/.degree. C.) to a positive slope of about 150 PPM/.degree. C., as plotted in FIG. 1. The most common dielectrics are the N4700, N3300, N2200, N1500, N750, N330, NPO (COG), P90, and P150. EIA-98 standard (Electronics Industries Association).
Per the EIA-98 standard, TC capacitors must have a quality factor Q of at least 1,000 (a percent dissipation factor % DF of 0.1). Q is the reciprocal of DF, or Q=1/DF or Q=1/(% DF X 0.01). To attain high Q values, though, the TC capacitor's microstructure density must exceed 95% of the theoretical value. In general, to attain a high density, an unfired TC capacitor would require firing at a temperature of at least 1,250.degree. C. But, such a high sintering temperature can adversely affect the fired TC capacitor:
a) Capacitors commonly use electrodes of 70% Ag/30% Pd alloy. This alloy has a solidus temperature at approximately 1,155.degree. C. and could therefore not be used if the sintering temperature is higher than 1,155.degree. C. This is problematic because in pure form, titanates or modified titanates, such as the alkaline earth metal titanates (pure or modified) or mixtures with zirconates or rare earth metal oxides, need sintering higher than 1,155.degree. C. to attain densities exceeding 95% of theoretical. PA1 b) In deriving TC capacitors, it is common to mix dielectric materials to achieve certain electrical and linear properties, such as a particular dielectric constant K or a TCC. For example, when positive TCC and negative TCC materials are mixed, a modified or shifted TCC results, such as mixing CaTiO.sub.3 (-TCC, N750) and MgTiO.sub.3 (+TCC, P110). Certain titanates or modified titanates, though, when mixed, can yield solid solutions rather than a multiple phase if sintered at 1,250.degree. C. or higher.
Prior art discloses certain glass modifiers and formers that lower sintering temperature to below 1,250.degree. C. For the most part, though, these modifiers/formers are based on heavy metal oxides, such as Bi.sub.2 O.sub.3, PbO, and CdO. While heavy metal oxides are useful to reduce sintering to below 1,155.degree. C., they are volatile and tend to reduce a dielectric's Q. Further, PbO and CdO are environmental hazards and the industry's trend has been to avoid them. And, Bi.sub.2 O.sub.3 can be reactive with Ag/Pd electrodes.
In U.S. Pat. No. 3,988,498, Maher, the present inventor disclosed a linear dielectric material of BaO.RE.sub.2 O.sub.3.TiO.sub.2 (RE is a rare earth metal). A glass of borate and silicate with a modifier Al.sub.2 O.sub.3 and a heavy metal oxide of PbO or Bi.sub.2 O.sub.3 were used to lower sintering temperature. CdO and ZnO were employed to adjust the TCC and glass melting temperature. This system, though, relied on heavy metal oxides.
In U.S. Pat. No. 4,533,974, Maher, the present inventor disclosed a Mg.sub.x Zn.sub.y TiO.sub.3 and CaTiO.sub.3 dielectric material. This system used a metal oxide flux of MgO.B.sub.2 O.sub.3, MgO.ZnO.B.sub.2 O.sub.3, or CdO.ZnO.B.sub.2 O.sub.3 flux to lower sintering to below 1,155.degree. C. The flux was borate dependent. The '974 also teaches that silicate and barium borate do not yield satisfactory results.
In U.S. Pat. No. 5,264,403, Abe et al. disclosed a system of BaO.RE.sub.2 O.sub.3.TiO.sub.2.Bi.sub.2 O.sub.3 and a glass of ZnO-B.sub.2 O.sub.3 -SiO.sub.2. The flux depends on the heavy metal oxide Bi.sub.2 O.sub.3 since, in the form in which it is added, the Bi.sub.2 O.sub.3 will act as a modifier. Further, the '403 treats metal oxides, such as BaO, as contaminants.
In contrast to the prior art, the present invention discloses a dual-component sintering flux based on zinc silicate and barium borate. This disclosed flux allows sintering of TC capacitors derived of positive and negative TCC dielectric materials at temperatures less than 1,155.degree. C. The fired densities are very high (often in excess of 95% of the theoretical value) and achieve the EIA-198 standard of % DF=0.1. Importantly, dielectric ceramic compositions based on the disclosed flux do not depend on glass modifiers of heavy metal oxides, such as Bi.sub.2 O3, PbO, or CdO.
Consequently, it is an objective of the present invention to provide TC capacitors and dielectric ceramic powders that have no glass modifers of heavy metal oxides, such as PbO, CdO, and Bi.sub.2 O.sub.3.
It is a further objective of the present invention to provide TC capacitors and dielectric ceramic powders that exhibit a high Q (or in excess of 5,000) and a very high density (or in excess of 95% of the theoretical value), where the unfired capacitor and dielectric compositions therefor can be sintered at less than 1,155.degree. C.
It is a further objective of the present invention to provide derivable dielectric ceramic powders that can exhibit formulated linear and electrical properties.
It is a further objective of the present invention to provide a dual-component sintering flux of barium borate and zinc silicate to enable sintering different dielectric ceramic powders and formulations at lower temperatures (less 1,155.degree. C.) yet provide a high Q (in excess of 5,000) and a high density (in excess of 95% of the theoretical value).
These and still further objectives will become apparent hereinafter.